1. Field of the Invention
The present invention relates to wavemeters for determining the wavelength of an optical beam.
2. Discussion of the Background Art
Wavemeters, i.e. devices for determining the wavelength of an optical beam, are well known in the art. While the main application of wavemeters is to determine the wavelength per se, they can also be applied for tuning a variable (or tunable) laser source. In the latter case, the optical beam can be regarded as being substantially monochromatic, so that the wavemeter fulfils the task of determining the precise wavelength of the variable laser source. In case that the measured wavelength does not coincide with the expected wavelength to be emitted by the variable laser source, adequate correction means might be provided for tuning the variable laser source to the expected or preset wavelength.
A first general principle for determining the optical wavelength makes use of changes in the characteristic properties of some materials in dependency of the wavelength. DE-A-3929845 discloses a dual detector with at least two detectors with different spectral sensitivity and a computer for determining the incident light wavelength from the photocurrent difference or ratio. A similar dual detector is disclosed in DE-A-3030210.
Variable wavelength filters are disclosed e.g. in JP-A-9089674 or GB-A 2288013. In JP-A-9089674, a wavelength detector includes a wavelength independent branching part into which a light beam is input. The branched output is passed to a wavelength-dependent branching part from which a pair of outputs is obtained. The outputs of the wavelength-dependent branching part are passed alternately to a photo-detector through an optical switch. The photo-detector detects the wavelength of the measured light from the ratio of the two outputs. According to GB-A-2288013, an optical fiber-based wavelength meter comprises two serially connected fused optical fiber couplers whose outputs are connected to a photo-detector with wavelength-sensitive gradients of coupling ratios of outputs being of opposite sign and outputs connected to a signal processor.
The above mentioned methods for determining the wavelength based on changes in the characteristic properties dependent on the wavelengths are generally only applicable for determining the wavelength of monochromatic signals and generally exhibit a poor accuracy.
A second possibility for determining the wavelengths applies the principles of diffraction or reflection at optical gratings. In the former case, light is emitted onto a grating, and the angle of the deflected beams indicates the wavelength. Although this solution generally allows determining the wavelength also of polychromatic optical signals, typical drawbacks are poor accuracy and the restriction in wavelengths of typically applicable detectors such as CCD-arrays.
A third possibility for determining the wavelength applies the principle of interference using interferometers such as Fabry-Perot or Fizeau. Those solutions are generally difficult to calibrate and applicable only in a narrow wavelength range.
A further interferometric solution makes use of the interferometric properties of etalons, i.e. plane parallel plates of glass or fused quartz with reflecting surfaces. Due to the interference at the surfaces within the etalon, the light beam transmitted through the etalon exhibits a wavelength dependent transmission characteristics, generally showing a sinus or cosines shape. As disclosed in EP-A-875743, the wavelength resolution can be significantly improved by combining two etalons with different phase dependencies on the wavelength. By providing a phase shift (in particular of n/2) between the two curves, the problem of the ambiguous turn points in the sinus-or cosines-like shapes can be avoided and a tangent relationship on the wavelength can be achieved. Instead of using two separated etalon elements, one birefringent element can be provided in combination with corresponding polarizing elements. Since the resulting transmission-over-wavelength characteristic is not unambiguous due to the periodicity of the curves, main applications are fine-tuning of variable laser sources, whereby the wavelength location is already roughly known and can therefor be assigned to a corresponding period of the curve.
DE-A-4114407 discloses an arrangement for determining the air path wavelength of a light source. Using an already known starting value for the air path wavelength, a current wavelength value is determined based on thermodynamic parameters of the ambient air.
U.S. Pat. No. 4,864,578 (Proffitt) discloses a scannable laser with integral wavemeter. The wavemeter comprises a fine wavelength read-out based on an ambiguous interferometric wavelength determination, and a course wavelength read-out making use of optically active quartz crystals. The amount of polarization rotation is measured as a sample light beam passes along its axis, and this is correlated to the wavelength of the light beam. Both wavelength read-outs are then utilized to determine the wavelength of the incoming light beam.
It is an object of the present invention to provide an improved wavemeter. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.
A wavemeter according to the invention for determining the wavelength of an incoming optical beam comprises a coarse-measuring unit and a fine-measuring unit. The coarse-measuring unit allows for unambiguously determining the wavelength of the incoming optical beam over a first wavelength range. The fine-measuring unit provides an ambiguous wavelength determination for the incoming optical beam, however with a higher accuracy than the accuracy of the coarse-measuring unit. Although the wavelength determination of the fine-measuring unit is ambiguous within the first wavelength range (e.g. since it provides a periodic wavelength dependency), it is provided to be unambiguous in each of a plurality of unambiguous wavelength ranges, whereby each of the plurality of unambiguous wavelength ranges is smaller than the first wavelength range.
An evaluation unit uses the result of the coarse-measuring unit for identifying one of the plurality of unambiguous wavelength ranges as a second wavelength range that covers the wavelength of the incoming optical beam. The result of the fine-measuring unit is then applied by the evaluation unit for determining the wavelength of the incoming optical beam within that identified second wavelength range.
In other words, the result of the coarse-measuring unit represents a first wavelength value with a first accuracy. However, since the measured value of the fine-measuring unit is generally ambiguous (i.e. plural wavelength values correspond to the measured value of the fine-measuring unit) but only unambiguous within each of the plurality of unambiguous wavelength ranges (i.e. within each of the plurality of unambiguous wavelength ranges, only one wavelength value corresponds to the measured value of the fine-measuring unit), the evaluation unit selectsxe2x80x94as the second wavelength rangexe2x80x94the one of the plurality of unambiguous wavelength ranges (of the fine-measuring unit) that covers the first wavelength value as provided by the coarse-measuring unit. With the information of the second wavelength range, the evaluation unit can then determine a second wavelength value as the one wavelength value corresponding to the measured value of the fine-measuring unit within the second wavelength range. The second wavelength value then represents the measuring result of the wavemeter for the wavelength of the incoming optical beam.
Thus, the wavemeter according to the invention allows determining the wavelength of the incoming optical beam within the first wavelength range and with the accuracy of the fine-measuring unit.
In case that the fine-measuring unit provides a periodic wavelength dependency, it is clear that in order to provide reasonable measuring results for the wavelength of the incoming optical beam, the periodicity (e.g. the wavelength range in which the fine-measuring units is unambiguous) of the wavelength-dependency of the fine-measuring unit has to be larger than the measuring fault or inaccuracy of the coarse-measuring unit.
In a preferred embodiment, the coarse-measuring unit employs one or more materials with a wavelength-dependency of characteristic properties, such as reflection and/or transmission. The applicable wavelength range can be controlled by selecting the respective materials as well known in the art.
Suitable materials, e.g. for providing a wavelength-dependency of a coating, can be for example MgF2, SiO, or CeF3. Preferred embodiments of the coarse-measuring unit are dielectric filters, wherein the reflectivity-and transmission-characteristics changes unambiguously within the wavelength range of interest. Dual detectors, wherein the incoming beam is split up and provided to detectors with different response characteristics over the wavelengths can be applied accordingly.
In one embodiment, the coarse-measuring unit employs a dielectric coating consisting of one or more layers of materials (e.g. MgF2, SiO, CeF3) with different refractive indices and thickness. The coating structure has to be designed, as well known in the art, to achieve unambiguous reflectivity-and/or transmission-characteristics within the wavelength range of interest.
In a more specific embodiment, the coarse-measuring unit comprises a glass plate with a dielectric coating on one side and an anti-reflection coating on the other side, thus representing a wavelength-dependent beamsplitter. Preferably, the dielectric coating provides a high wavelength-dependent reflection/transmission characteristics, and the anti-reflection coating is optimized for providing minimum reflection to avoid light disturbances coming from the second surface that might falsify the transmission and reflection characteristics of the dielectric coating. The beams separated by the wavelength-dependent beamsplifter are preferably launched to photodiodes for measuring the characteristic properties, such as reflection and/or transmission of the dielectric coating. The applicable wavelength range can be controlled by selecting the respective materials.
A preferred embodiment of the fine-measuring unit makes use of the interferometric principle, such as the Fizeau interferometer or uses a combination of different etalons (which can be also realized based on polarization effects)as disclosed in detail in the aforementioned EP-A-875743. Those interferometric units generally provide a periodic dependency over the wavelength, but exhibit a higher resolution than the units employing wavelengths dependent material properties. However, due to the periodicity of the wavelength dependency from the interferometric measurement, the periodicity of the wavelength-dependency of the fine-measuring unit should be larger than the measuring fault or inaccuracy of the coarse-measuring unit.
In a further preferred embodiment, the wavemeter according to the invention further comprises an absolute-measuring unit making use of unambiguous wavelength properties, e.g. absolutely known transmission features as provided e.g. by gas absorption cells. In such gas absorption cells, the incoming light is passed through a gas cell acting as an optical filter having absolutely known transmission features, preferably known absorption lines of the gas. Such filters are described e.g. in US-A-5,780,843 for controlling high accuracy tunable laser sources.
Each measuring unit (i.e. the coarse-measuring unit, the fine-measuring unit, and, if available, the absolute-measuring unit) of the wavemeter is coupled to the evaluation unit receiving measuring signals (e.g. optical power values) from each of the measuring units, and determining the wavelength of the incoming optical signal therefrom. This can preferably be done using calibrated look-up tables for the coarse-and fine-measuring unit. One or more correction (offset) values coming from the absolute-measuring unit can be applied in addition as will be described later.
Since the absolute-measuring unit generally only allows unambiguously determining discrete wavelengths but does not for a continuous spectrum, the absolute-measuring unit is preferably provided for reference measurement(s). The wavemeter thus provides a corrected output (representing the wavelength(s) of the incoming beam(s))by using the unambiguous wavelength properties of the absolute-measuring unit for calibrating the wavemeter. The reference measurement is preferably applied for calibrating the wavemeter before an actual measurement.
Preferably for (absolutely) calibrating the wavemeter, an input signal is swept over a wavelength range wherein the absolute-measuring unit has at least one of the known absolute wavelength characteristics. By analyzing the measured transmitted power of the absolute-measuring unit together with the wavelength-results derived from the coarse-measuring unit and/or the fine-measuring unit, a relation between the absolutely known transmission features and the derived wavelength-results can be established. This can result in one or more correction (offset) values relating to an e.g. factory based calibration of the wavemeter.
Since a reference measurement employing the absolute-measuring unit generally requires a wavelength sweep, it is clear that a continuous calibration is only applicable as long as a wavelength sweep is applied at all, and as long as the wavelength sweep covers at least one of the unambiguous wavelength properties of the absolute-measuring unit.
The necessity of calibration generally depends on the wavelength stability and/or wavelength characteristics of the coarse-measuring unit and/or the fine-measuring unit, which may be influenced by temperature, mechanical shock or aging.
Thus, the invention provides a wavemeter allowing to assign with a higher precision and accuracy a wavelength to an incident light beam. The applicable wavelength range of the wavemeter can be adjusted by selecting or designing the wavelength-dependencies of the coarse-measuring unit and/or the fine-measuring unit.
The term xe2x80x98monochromatic lightxe2x80x99 in the context of this invention shall mean small-bandwidth light with substantially one dominating characteristic wavelength.
It is clear that the invention can be partly or entirely embodied by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit. In particular, software programs might be applied by the unit and for controlling a wavelength sweep of a light source.